modify validity checks

[ajrominger]

It would be best to get rid of sample_valid and instead ensure that ranges provided by the user are valid before even beginning to sample.

So is_valid should be modified in two ways

1. it should cause ModelPrior to error out if ranges are supplied that are invalid
2. it should check ranges supplied to IntDistribution and FloutDistribution rather than checking individual parameter values

To see the valid ranges for each parameter see #7